Linearity refers to the ability of an RF amplifier portion of an RF transmitter to amplify without distortion. Desirably, distortion is held to a minimum and RF amplifiers are as linear as possible so that the RF transmitter will broadcast the intended signal, confined within the intended spectral band, and refrain from interfering in other spectral bands.
Unfortunately, typical RF transmitters are teeming with causes for the distortion that invariably appears to some degree in the signals generated by RF amplifiers. For example, the transfer curve of the RF amplifier itself may simply fail to be linear over the entire signal range. In-band distortion may result from imbalances between quadrature components of complex signals being processed. Bias feed networks may provide varying bias signals or otherwise interact with a signal to be amplified in a manner that alters the spectral characteristics of the signal. And, predistorters which attempt to introduce a canceling distortion prior to amplification in an RF amplifier often use corrupted feedback signals derived from an output of the RF amplifier in order to determine what sort of canceling distortion should be introduced. All these sources of distortion operate in concert with one another causing the joint effect to be difficult to adequately address.
Thermal effects represent yet another cause for the distortion that appears in signals generated by RF amplifiers. Thermal effects refer to the distortion resulting from operating the RF amplifier at different temperatures. Thus, if an RF transmitter is perfectly configured to minimize distortion when its RF amplifier operates at one temperature, as soon as the RF amplifier operates at a different temperature, the RF transmitter will no longer be perfectly configured to minimize distortion.
Thermal effects operate in concert with other causes of distortion, but differ from the other causes in that they become evident over an entirely different time scale. Most of the other causes of distortion produce their full distorting effect within the short span of time required for a communication signal to propagate through the RF transmitter, if not instantly. Often, a significant consequence of the other distorting effects is spectral corruption of the amplified signal, such as the generation of unwanted intermodulation products and spectral regrowth. The spectral corruption is often mitigated by spectrally processing the communication signal, but the wider the bandwidth of the communication signal, the more difficult the spectral processing task becomes.
Thermal effects produce their distorting consequences more slowly. While spectral corruption may also result from thermal effects, the spectral corruption may be a more indirect result. For example, an RF amplifier may heat up as the RF amplifier amplifies a greater magnitude signal. But the heating occurs gradually and in proportion to the total energy consumed over a longer period of time rather than to any particular instantaneous signal power level. The heating typically influences an RF amplifier by causing the RF amplifier to exhibit a gain variation versus higher temperatures.
Conventional techniques for linearizing RF amplifiers have addressed thermal effects. But conventional techniques typically attempt to compensate for thermal effects using techniques or extensions of techniques that also compensate for the more instantaneous causes of distortion. These techniques can interact to each other's detriment. The result of using techniques or approaches to reduce thermal effects that are commonly used to reduce other causes of distortion is that conventional RF transmitters tend to produce excessive amounts of distortion.